1. Field of the Invention
The present invention relates to a system for recording and playing back information, and more specifically to a system which utilizes a recording medium having magneto-optical characteristics.
2. Description of the Prior Art
In recent years, recording systems have been proposed wherein information is recorded and played back by using a so-called "magneto-optical disc memory". The magneto-optical disc memory includes a base which is made of a transparent material such as glass or a plastic, and a so-called "vertical magnetization film" which is formed thereon and is used as the recording medium having magneto-optical characteristics.
An amorphous alloy thin film of several microns thick is an example of a "vertical magnetization film" in which the direction of magnetization is perpendicular to the surface of the film.
Recording of information on the amorphous thin film takes place in a manner such that the orientation or the direction of magnetization of portions of the amorphous alloy thin film, which is initially uniformally magnetized in a preselected direction, is turned over by heating those portions above the Curie temperature or a compensation temperature while applying a relatively weak bias magnetic field. Generally, a laser beam digitally modulated by a signal to be recorded is applied on the amorphous thin film to heat those portions.
More specifically, the amorphous alloy thin film, which was previously downwardly (corresponding to the binary "0" signal) magnetized, is subjected to a weak upwardly biased field, while only those portions of the thin film on which the "1" signal is to be recorded are applied with a laser beam so as to raise the temperature of those portions above the Curie temperature or a compensation themperature, thereby causing a change in the direction of magnetization. Once the direction of magnetization is changed by the above-described process, it will not be changed by the bias magnetic field under the condition of room temperature.
Gadlinium Cobalt (GdCo) and Gadlinium Terbium Iron (GdTbFe) are examples of amorphous materials for the thin film. In the case of Gadlinium Terbium Iron, the power level of the laser beam required to change to magnetization direction is less than several m.omega., wherein the density of the recorded information is around 5 million bits per square centimeter.
The read out of information recorded on the magneto-optical disc memory takes place by applying a readout laser beam on the surface of the vertical magnetization film and the recorded signal is reproduced by utilizing the magnetic Kerr effect, which is an interaction between the magnetization of a substance and a light beam. When a linearly polarized laser beam is reflected by the magnetized thin film, the plane of polarization is rotated in a direction which is determined by the direction of the magnetization. Therefore, the information recorded on the vertical magnetization film is read out by detecting the direction of rotation of the plane of polarization of the reflected laser beam by means of an analyser.
In this way, recording and reading out of information take place in a recording and playing back system which has a magneto-optical disc memory as the recording medium.
This type of recording and playing back system, however, tends to be large-scaled because of the provision of both of a recording optical system and a playing back optical system. Furthermore, it may result in an increase in the cost of those systems.